1. Field of the Invention
This invention relates generally to a method for providing power distribution in a fuel cell system and, more particularly, to a fuel cell system that provides a high compressor power and a low cathode output pressure to improve up-transient power responses.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The dynamic power of a fuel cell system is limited. Further, the time delay from system start-up to driveability and low acceleration of the vehicle may not be acceptable. The voltage cycles can increase the stack durability. These drawbacks can be minimized by using a high voltage battery in parallel with the fuel cell stack. Algorithms are employed to provide the distribution of power from the battery and the fuel cell stack to meet the requested power.
Some fuel cell vehicles are hybrid vehicles that employ an electric energy storage system (EESS) in addition to the fuel cell stack, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The EESS provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power through a DC voltage bus line to an electric traction system (ETS) for vehicle operation. The EESS provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the EESS at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
Acceptable time periods for automotive applications from idle power to full power is on the order of two to three seconds. Currently, fuel cell vehicles are not able to provide power up-transients as quickly as desired. Generally, the up-transient response limitations for fuel cell systems occur because the fuel cell stack cannot receive cathode air fast enough. The compressor itself is able to ramp up fast enough provided that it receives adequate power. However, it is the ability to provide the power to the compressor that limits the ability of the compressor to provide airflow to the fuel cell stack fast enough. Part of the problem is cyclical in that the compressor typically receives its power from the fuel cell stack, and the fuel cell stack power is generally too low at low stack power to provide high compressor speeds.
The high voltage battery typically provided in a hybrid fuel cell vehicle can help provide compressor power during power up-transients to provide the requested ETS power quick enough. However, the battery has limitations in that typically the battery will not be allowed to fall below a predetermined minimum state-of-charge (SOC).